Temporal changes of site response during the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

Abstract

The recent M w 9.0 off the Pacific coast of Tohoku Earthquake and its aftershocks generated widespread strong shakings as large as ∼3000 Gal along the east coast of Japan. Here we systematically analyze temporal changes of material properties and nonlinear site response in the shallow crust associated with the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KiK-Net. We compute the spectral ratios of windowed records from a pair of surface and borehole stations, and then use the sliding-window spectral ratios to track the temporal changes in the site response of various sites at different levels of peak ground acceleration (PGA). Our preliminary results show clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites with PGA from 600 to 1300 Gal. In the site MYGH04 where two distinct groups of strong ground motions were recorded, the resonant frequency briefly recovers in between, and then followed by an apparent logarithmic recovery. We investigate the percentage drop of peak frequency and peak spectral ratio during the Tohoku main shock at different PGA levels, and find that at most sites they are correlated.

Long-term change of site response after the M W 9.0 Tohoku earthquake in Japan

“…Here we conduct a follow-up study of Wu and Peng (2011) utilizing the newly available strong motion data recorded by the Japanese Digital Strong-Motion Seismograph Network KiK-Net from 15 months before to 5 months after the Tohoku main shock. The KiK-Net is operated by National Research Institute for Earth Science and Disaster Prevention and consists of 659 pairs of surface/borehole strong-motion seismometers (Aoi et al, 2000).…”

“…Following Wu and Peng (2011), we analyze data recorded by six stations: FKSH10, IBRH12, IBRH15, IBRH16, MYGH04, and TCGH16 (Fig. 1).…”

“…1). We focus on these six stations mainly because the observed temporal changes in resonance (peak) frequencies at these stations are much clearer than those at other stations (Wu and Peng, 2011), allowing us to better quantify long-term subtle temporal changes after the Tohoku main shock. In addition, the velocity contrasts between the surface soil layers and the underlying bedrocks are relatively stronger than those at other nearby KiK-Net stations (Fig.…”

“…The Tohoku main shock and its aftershocks generated widespread strong shakings as large as ∼3000 Gal along the east coast of Japan. Wu and Peng (2011) found clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites and correlation of resonance (peak) frequency and peak ground acceleration (PGA) during the main shock. Here we follow that study and systematically analyze long-term changes of material properties in the shallow crust from one year before to 5 months after the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KiK-Net.…”

“…Several recent studies have shown evidence of soil nonlinearity at various sites in Japan during the Tohoku main shock (e.g., Bonilla et al, 2011;Nakata and Snieder, 2011;Wu and Peng, 2011). In particular, both Wu and Peng (2011) and Nakata and Snieder (2011) found apparent logarithmic recovery after the Tohoku main shock. However, the soil properties still did not recover to the level before Tohoku earthquake at the time when these studies were conducted.…”